1. Field of the Invention
The present invention relates to error checking and manipulation of a design layout, and more particularly to computer aided design layout and design rule verification of an integrated circuit design layout, and use thereof for circuit fabrication.
2. Background of the Invention
Design of an electronic circuit, for example, an integrated circuit (IC), is a complicated and time consuming process. FIG. 1 illustrates a typical design flow 80 of an integrated circuit device from conception through the generation of a fabrication ready design layout. Generally, design flow 80 commences with defining the design specifications or requirements, such as required functionality and timing, at step 82. The requirements of the design are implemented, for example, as a netlist or electronic circuit description, at step 84. The implementation can be performed by, for example, schematic capture (drawing the design with a computer aided design tool) or more typically, utilizing a high level description language such as VHDL, Verilog(copyright) and the like. The implemented design is simulated to verify design accuracy, at step 86. Design implementation and simulation are iterative processes. For example, errors found by simulation are corrected by design implementation and re-simulated.
Once the design is verified for accuracy with simulation, a design layout of the design is created, at step 88. The design layout describes the detailed design geometries and the relative positioning of each design layer to be used in actual fabrication. The design layout is very tightly linked to overall circuit performance (area, speed and power dissipation) because the physical structure defined by the design layout determines, for example, the transconductances of the transistors, the parasitic capacitances and resistances, and the silicon area which is used to realize a certain function. The detailed design layout requires a very intensive and time-consuming design effort and is typically performed utilizing specialized computer aided design (CAD) or Electronic Design Automation (EDA) tools.
The design layout is checked against a set of design rules in a design rule check (DRC), at step 90. The created design layout must conform to a complex set of design rules in order, for example, to ensure a lower probability of fabrication defects. The design rules specify, for example, how far apart the geometries on various layers must be, or how large or small various aspects of the layout must be for successful fabrication, given the tolerances and other limitations of the fabrication process. A design rule can be, for example, a minimum spacing amount between geometries and is typically closely associated to the technology, fabrication process and design characteristics. For example, different minimum spacing amounts between geometries can be specified for different sizes of geometries. DRC is a time-consuming iterative process that often requires manual manipulation and interaction by the designer. The designer performs design layout and DRC iteratively, reshaping and moving design geometries to correct all layout errors and achieve a DRC clean (violation free) design.
Circuit extraction is performed after the design layout is completed and error free, at step 92. The extracted circuit identifies individual transistors and interconnections, for example, on various layers, as well as the parasitic resistances and capacitances present between the layers. A layout versus schematic check (LVS) is performed, at step 94, where the extracted netlist is compared to the design implementation created in step 84. LVS ensures that the design layout is a correct realization of the intended circuit topology. Any errors such as unintended connections between transistors, or missing connections/devices, etc. must be corrected in the design layout before proceeding to post-layout simulation, step 96. The post-layout simulation is performed using the extracted netlist which provides a clear assessment of the circuit speed, the influence of circuit parasitics (such as parasitic capacitances and resistances), and any glitches that can occur due to signal delay mismatches. Once post-layout simulation is complete and all errors found by DRC are corrected, the design is ready for fabrication and is sent to a fabrication facility.
As electronic circuit densities increase and technology advances, for example, in deep sub-micron circuits, skilled designers attempt to maximize the utilization of the design layout and manufacturability and reliability of the circuit. For example, the density of a layer can be increased, additional vias added to interconnection areas, and the like. Creation of a design layout and performing DRC become critical time consuming processes. Performing a DRC and manipulation of the design layout often requires manual interaction from the designer. More reliable and automated techniques for improving the design layout are consistently desired.
In a modern semiconductor design technology, many metal layers are used to implement interconnections throughout an integrated circuit. For some integrated circuits, one or more polysilicon (poly) layers are also used to implement interconnections. Vias are used to connect from one such metal or polysilicon layer to another metal or polysilicon layer. For example, a via may be used to connect a feature (i.e., a design geometry) on each of two metal layers. The lower one of the two layers is referred to as the landing metal layer and the upper one of the two layers is referred to as the covering layer. A via between a landing metal layer mtx and the covering metal layer mtx+1 is usually referred to as a vx via (i.e., using the same subscript designation as the landing metal layer).
Most design technologies include via enclosure rules to ensure that both the landing metal and the covering metal enclose the via by a certain amount. In other words, such an enclosure rule ensures that each metal layer overlaps a via with a certain amount of extra metal, to ensure that the via provides a good connection between the two metal layers once fabricated. The design rule specifying the extra amount of metal around each via may be referred to as a metal enclosure of a via design rule, and at times simply as a via enclosure design rule.
In a modern semiconductor design technology, especially for a deep sub-micron design, poly and metal layers which are used to implement connections through vias apply different via enclosure rules depending on the width of the metal or poly in the vicinity of the via. When a via is placed in a wide metal area, it may need more metal enclosure than that of a via which is placed in a narrower metal area. When a via is partially in wide metal area and partially in non-wide metal area, it may need different metal enclosure in each different area. In general, as the design technology advances, more and more wide classes of metal features may be used in a design layout, having different metal enclosure design rules for each wide metal class. Automated techniques are desirable for interacting with a design layout which includes multi wide class objects without causing false errors (or false passes), and additional improvements are needed.
In a deep sub micron semiconductor design, vacancy concentration sometimes causes failure on single vias which are located at the center part of a wide landing metal or covering metal geometry, known as an isolated via. It has been observed that putting redundant vias between the same landing and covering metal as the isolated via, results in the vias sharing the vacancy load, so the risk of via failure is reduced. Vias which are close together (i.e., very proximal vias) and vias which are close to the edge of the metal geometry may also benefit from alteration of the stress gradients responsible for vacancy transport, and likewise reduce the risk of via failure. To avoid isolated vias, a redundant via design rule check is preferably performed on design layouts. In exemplary embodiments, a redundant via design rule check preferably ensures that for vias placed within a connection area of a metal feature (or within a localized region of a larger metal geometry) that is both greater than a certain width and greater than a certain area in size, the vias are both sufficient in number and/or proper in their location. Such redundant via checks may be performed on landing metal, on covering metal, or both.
In an exemplary embodiment, a redundant via design rule check requires that for a metal piece (or region of a piece) which is both greater than a certain width and greater than a certain area in size, any connection area within it (i.e., an area having at least one via connecting it to another metal layer) should contain at least a certain number of vias, some of which should be close to the landing metal edge or close to each other. The redundant via design rule may apply differently on different classes of objects which are defined by both minimum width and minimum area. In some embodiments, such a redundant via test may be waived for any connection area having at least a certain minimum number of vias therewithin.
The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail; consequently, those skilled in the art will appreciate that the summary is illustrative only and is not intended to be in any way limiting. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, will become apparent in the non-limiting detailed description set forth below.